Short, low power pulses can be generated throughout the visible, near infrared and ultraviolet regions.
Progress in short pulse generation in the visible has traditionally resulted from improvements in actively or passively mode-locked lasers. At other frequencies, however, this has not been the case.
U.S. Pat. No. 3,720,884, issued Mar. 13, 1973 to Kelley et al, discloses a technique for compressing optical pulses by chirping the pulse in a Kerr cell, and passing these pulses through a dispersive delay line to achieve the required compression. This technique is limited to near infrared or higher frequencies.
U.S. Pat. No. 3,979,694, issued Sept. 7, 1976 to Goldhar et al, teaches the generation of short pulses having a duration of between 0.1 and 0.5 nanoseconds by using a gaseous breakdown switch and a hot gaseous absorption cell in conjunction with a CO.sub.2 transverse excitation laser. The apparatus and method taught by Goldhar et al does not, however, provide for the possibility of generating pulses as short as picosecond or femtosecond pulse durations, but rather produces pulses having a duration of the order of nanoseconds by a technique which involves the spectral filtering of the output of a gas breakdown switch using a hot linearly absorbing gas cell or a spectral filter formed by a tandem grating monochromator of the type which is a tandem dual-slit monochromator.
There is a need for the generation of extremely short infrared pulses in the fields of multiphoton chemistry, plasma physics, and nuclear fusion. This need is not satisfied by any known prior art apparatus or method.
The concept of efficiently compressing optical pulses by impressing a frequency chirp on the pulse and then passing the pulse through a linear dispersing medium for pulse shortening, was proposed many years ago. Kerr liquids such as CS.sub.2 can provide such a chirp. However, in liquids, self-focussing is normally associated with frequency chirping. It has remained for the development of low loss, single-mode optical fibers to make the process reproducible. Thus, it is only with the recent improvement in optical fibers that optical pulse compression has begun to have an important impact on the development of picosecond/femtosecond sources.
Low-loss, single-mode optical fibers are not available in the mid-infrared. Even if they were, the wavelength scaling of self-phase modulation in Kerr-like materials would make this approach to optical pulse compression somewhat less attractive. Plasma production can provide an alternative technique for chirping pulses; a technique especially suited to the infrared.
In Kelley et al, supra, a frequency chirp is impressed on an optical pulse as a result of the nonlinearity of the refractive index (.eta.=.eta..sub.o +.eta..sub.2 E.sup.2) associated with different optical intensities. That is, the phase velocity is dependent on the intensity and, therefore, on the temporal position within the pulse. Any process that produces an index of refraction that is different for different temporal positions in the pulse will, likewise produce a frequency chirp.
This invention shows a technique for compressing optical pulses that is particularly suited to the infrared, although not necessarily restricted to the infrared. It is also particularly suited to high power pulses, but not restricted to them.